Daytime low‐altitude quasi‐periodic echoes at Gadanki: Understanding of their generation mechanism in the light of their Doppler characteristics

Abstract

In this paper, we investigate the genesis of low‐altitude quasi‐periodic (LQP) E region echoes observed during daytime by the Gadanki radar. We show that such echoes reflect much wider Doppler spectra compared to that of typical daytime low‐altitude E‐region echoes. Further, the Doppler spread resembles that of meteor induced spread echoes. Considering that the meteor spread echoes are generated by the gradient drift instability on the sharp density gradient associated with the plasma trail, we invoke plasma density gradient and a similar physical process to account for the LQP spread echoes. Taking into account all the pertinent issues related to the formation of plasma density structures in the collision dominated lower E region, we surmise that the required plasma density structures are generated by a weakly driven Kelvin‐Helmholtz instability. These structures get polarized by the background electric field or neutral wind and become gradient drift unstable manifesting LQP echoes.

Tropical mesopause: Is it always close to 100 km?

“…Another relevant aspect and frequently observed phenomena is the observation of VHF (MST) radar echoes from the magnetic filed‐aligned irregularities (FAI) in the altitude region of 88–100 km [ Krishna Murthy et al , 1998; Patra et al , 2006] over this station. Although their structures were observed to have features of tidal ion layer commonly observed and reported for Arecibo [e.g., Mathews , 1998], radar echoes from E region FAI extending down to altitude as low as 88 km were shown to be puzzling [ Patra et al , 2006, 2009]. Patra et al [2009] provided a detailed analysis on the genesis of ultralow altitude echoes and advocated that they are generated by weak Kelvin‐Helmholtz instability (KHI).…”

“…Although their structures were observed to have features of tidal ion layer commonly observed and reported for Arecibo [e.g., Mathews , 1998], radar echoes from E region FAI extending down to altitude as low as 88 km were shown to be puzzling [ Patra et al , 2006, 2009]. Patra et al [2009] provided a detailed analysis on the genesis of ultralow altitude echoes and advocated that they are generated by weak Kelvin‐Helmholtz instability (KHI). Thus, it is prudent to consider the temperature structures, since they play an important role on the excitation of KHI.…”

Review of ionospheric irregularities and ionospheric electrodynamic coupling in the middle latitude region

“…Driven by KHI, E S structure is destabilized by collision between plasma and neutral particles, and then evolves into the K‐H structure, resulting in QP echoes in radar maps. Correlative studies have been performed by Sripathi et al (2003) and Patra et al (2009). A 2‐D numerical model of the evolution of E S in KHI modulation is shown in Figure 9 (Bernhardt, 2002).…”

First simultaneous lidar observations of sodium layers and VHF radar observations of E-region field-aligned irregularities at the low-latitude station Gadanki

“…Coming to the low-latitude observations of E-region irregularities, investigations suggested that the low latitude Eregion irregularities are closely linked to the tidal and gravity wave dynamics (e.g., Patra et al, 2005Patra et al, , 2009Venkateswara Rao et al, 2008). Patra et al (2009), based on their investigation, surmised that metallic ions must be playing a crucial role in the layer formation and thus in the instability process.…”

“…Patra et al (2009), based on their investigation, surmised that metallic ions must be playing a crucial role in the layer formation and thus in the instability process. No experimental proof, however, could be provided so far.…”